P3.2 Flashcards
How does the resistance of some components change?
A metal is made up of positively charges ions arranged in a regular pattern. The ions are formed when electrons leave the outer shell of metal atoms. These delocalised electrons are free to move through the structure of the metal. Resistance is produced when electrons collide with the ions in the lattice. This explains why rage resistance of some components changes with current.
Voltmeter in a series circuit
Voltmeters measure the potential difference across a component. They are connected in parallel to the component.
A voltmeter measures the rise in potential across a battery/cell. It measures the drop in potential across a component
Potential difference in parallel
The potential difference across each component is the same as the battery.
What can you do if the bulb doesn’t light when building circuits?
- connect the bulb to the battery, if it doesn’t work, try another battery.
- if that doesn’t work, try changing the bulb.
- when they work, check each bulb you are using.
- if the bulb still doesn’t light, replace one lead at a time to check which one is faulty, and once you find it replace this one.
- you can also check that the ammeter’s negative terminal is nearest to the negative terminal of the battery.
How does current change and ohms law
Current depends on the potential difference of the cell or battery and the resistance of the components. It can only be changes by changing the potential difference or resistance.
Ohm’s law: current is proportional to the potential difference if the temperature does not change.
Changing resistance
A metal is made up of positive ions arranged in a regular pattern. The ions are formed when electrons leave the outer shell of metal atoms. These delocalised electrons are free to move through the structure of the metal. Resistance is produced when electrons collide with the ions in the lattice.
What affects resistance
A thinner wire will have a greater resistance than a thicker wire as there is a greater chance of the electrons colliding with the positive metal ions. The same is true with a longer wire.
Variable resistor
Variable resistors can be used to change the amount of wire the current flows through.
Characteristic graph
Characteristic graph is one that has current against potential difference. You collect data for the graph by taking measurements of the current flowing through a component for different potential differences. Then you reverse the battery or power supply connections and repeat the measurements
Linear circuit element
Component that has a resistance that does not change. For example resistance wire or a fixed resistor.
Non-linear circuit element
Component that has a resistance that does change. For example a lamp.
—> The electrons in a wire collide with the ions in the wire. The ions vibrate more as the wire gets hotter. As that happens there are more collisions. The wire heats up ore. The current increases as the potential difference increases, but at a slower rate.
Non-linear circuit element - diode
Diodes only let current flow one way. Some emit light - light emitting diode.
As potential difference is applied in the forward direction, very little current follows then rapidly increases. Reversing the potential difference means no current flows as diodes only let current flow in one direction.
What is a thermistor
The resistance of a thermistor changes with temperature. It is made of a semiconducting material (silicon). The electrons present in the atoms of a semiconductor do not need much energy to escape from the atom to form the current.
What happens when the thermistor is heated?
As the thermistor is heated, electrons gain enough energy to escape from the atoms in the semiconductor so the resistance drops.
At the same potential difference:
- at low temperatures, the current is a small so the resistance is high.
- at high temperatures, the current is high so the resistance is low.
What is a LDR
The resistance of. Alight-dependant resistor changes with light intensity. It is made of a semiconductor. Light causes electrons to be released into the circuit to increase the current.
What happens when the light intensity of an LDR changes?
As the light intensity increases, more electrons are released in the semiconductor and the resistance decreases.
What would happen to current and net resistance if another identical lamp is added in series/parallel? What to remember?
In series:
- the current decreases
- the net resistance increases
In parallel:
- the current increases
- the net resistance decreases
REMEMBER IN SERIES:
The current is the same everywhere, the p.d.s across each component add up to the p.d across the battery, V = IR
REMEMBER IN PARALLEL:
The p.d is the same across each loop, the current in each loop adds up to the current near the battery, V = IR
Power formulae
In an electrical circuit :
- the current is the rate of flow of charge - it tells you the amount of charge flowing through a component per second
- the potential difference tells you the energy transferred by each charge
Potential difference = energy transferred / charge
Current = charge / time
Power = potential difference x current
Energy transferred = power x time
Power - current^2 x resistance
